1. Field of the Invention
The present invention relates to optical transmission systems, and more particularly, to an optical transmission system that performs optical transmission control.
2. Description of the Related Art
The demand of international telecommunications is rapidly expanded by globalization of business, the spread of the Internet and so on. In that situation, an optical submarine transmission system is important together with satellite communications, and early realization of an economical, large-capacity optical submarine transmission system is strongly desired.
In the optical submarine transmission system, optical fiber cables are laid to the sea floor, and are connected through repeaters so as to obtain an extremely long transmission distance with optical amplifying. Further, the optical submarine transmission system is demanded to have the severest reliability because a fault that occurs underwater needs a huge amount of cost and time to repair the fault. Therefore, it is required to provide the system with a fault detecting function of locating a fault definitely.
In the fault detection control, an end station on the land sends repeaters an optical instruction for monitoring the operating condition thereof. Each repeater receives the instruction and monitors its own operating condition, then sending back a response to the end station. In this manner, the end station and the repeaters communicate with each other to monitor the condition of optical submarine transmission.
The conventional repeaters employ an erbium-doped optical fiber amplifier (abbreviated as EDFA) for optical amplification. The response signal sent by the EDFA repeater includes response information that is modulated onto a main signal that is the output of a pumping laser diode for exciting the EDFA.
Even if a fault such as breakdown occurs in the optical fiber cable to cause the optical main signal to be lost, the EDFA repeater continues to perform the monitor control because the response signal can be sent by modulating an amplified spontaneous emission (ASE) emitted by the EDFA itself, which is an amplifying medium.
Recent optical communication systems employ an optical fiber amplifier (Raman amplification), which utilizes a non-linear optical phenomenon that occurs within the optical fiber, called Raman amplification. This utilizes a physical effect such that light having a different wavelength from that of an incident light is scattered because of a vibration effect within a substance. Amplification is implemented by applying a strong pump light so as to travel down the whole optical fiber transmission medium. The Raman amplification does not limit the amplifiable range.
Application of the Raman amplification to the repeater enables a longer optical fiber to be laid and increases the intervals at which repeaters are arranged.
The response signal that represents the operating condition of the Raman amplification repeater can be sent therefrom in the same manner as that for the EDFA repeater. That is, response information is modulated onto the output of the pump laser diode that excites the optical fiber so that the main signal is modulated.
However, the Raman amplification repeater that does not have EDFA has a problem described below. If a fiber cable fault occurs at a position close to the repeater, the amplifying medium is no longer available. This results in loss of ASE and means for sending the response signal to the end station by modulation of ASE. As a result, the monitor control is lost after the fiber fault occurs.